The present invention relates to networked systems composed of a plurality of devices clustered for the exchange of data and control messages formatted according to predetermined protocols and, in particular, to such systems where inter-device communication is via wireless link. The invention further relates to devices for use in groups or clusters to form such systems.
Networked interconnection of devices is well known, for example in hi-fi systems or security systems having detectors, a control panel and one or more alarm sounders. A development has been the so-called home bus systems where a greater variety of products have been linked with a view to providing enhanced overall functionality for example domestic audio/video apparatus coupled with a home security system and the use of telephone. An example of such a home bus system is the domestic digital bus (D2B), the communications protocols for which have been issued as standard IEC 1030 by the International Electrotechnical Commission in Geneva, Switzerland. The D2B system provides a single wire control bus to which all devices are interfaced with messages carried between the various devices of the system in a standardised form of data packet.
Even with a single wire link from device to device, the amount of inter-connection can become unwieldy leading to installation problems for the user, especially where a cluster of devices to be linked is spread across two or three rooms. In order to get around some of these problems, the use of radio frequency or infra-red communications has been proposed to replace the wired links with a central base station receiving messages from one device and forwarding to their destination device. The need for a base station introduces its own problems, not the least of which is the fact that each message must be transmitted twice, namely from its source to the base station and from the base station to its target. In terms of consumer systems, the user is constrained to buy the base station first and, being the most complex part of the system, it will generally be the most expensive. There is also the problem common to all such centralised systems, namely that if the base station fails, the whole system becomes unusable.
While the use of wireless communications removes some of the physical constraints on system installation, locations such as the home environment do not provide an ideal communications space. Where wireless communication between stations is via infra-red link, each communicating unit needs to have direct line of sight to the others, and data may be lost or unacceptably delayed simply through a user having walked across a room and interrupted the line of sight between two communicating devices. Radio frequency (RF) communications are an improvement in this respect, in that they can pass through walls, doors and ceilings: this makes RF communication particularly suited where clusters of devices spread across two or three rooms. While RF communication is less susceptible than IR to line of sight blockage, it does have a greater vulnerability to signal reflection leading to multi-path errors and garbling of data when received.
It is therefore an object of the present invention to provide a wireless communications system, and devices for use in the same, having reduced vulnerability to signal loss arising from line-of-sight blockage and/or signal multi-path effects.
In accordance with the present invention there is provided a wireless communications system comprising a plurality of nodes clustered for the exchange of data and control message packets. Each node has a unique address within the system, and is arranged to ignore received messages not containing their own node address. Each node is arranged to generate and transmit an acknowledgement message to an originating node of a data packet received directly. Each node also has a store holding a node address for each other node of the system. Each node is arranged such that, on determining a lack of receipt of an acknowledgement message for a message packet transmitted to a target node, a further node address is selected from the store and said message packet is transmitted to a further node as a re-routed message packet, Each node is configured to identify the original target of a received re-routed message packet and to transmit the re-routed message packet to its original target.
By use of the node table carried (in full or partial form) by each of the devices making up a cluster, alternative routes from originating to target nodes may be selected dynamically in order to minimize the effects of short-term channel path interference or blockage. Such a re-routing procedure, not applicable to systems where all messages are routed through a base-station, makes use of the physical separation of the nodes or devices to provide a physical detour around blockages.
Suitably, each node is configured to generate and transmit an acknowledgement message to the transmitting node of a successfully received re-routed message packet, with each node re-sending a message packet a predetermined number of times if it does not receive an acknowledgement within an expected period. Re-routing need not be limited to the use of one intermediate node: each node may be operable to select, on determining the lack of a received acknowledgement message for a re-routed message packet, a still further node address from store, other than those of the target node and message originating node, and to transmit the message packet to this still further node as a re-routed message. Thus, an intermediate node unable to contact the target node may simply pass the message packet on to another node in the cluster to attempt to contact the target.
Where messages may be re-routed more than once, means are suitably provided to limit the number of re-routed transmissions of a message packet to a pre-determined number of re-routed transmissions. This may be achieved by each message packet carrying a data field reserved to indicating unsuccessful transmission of that message packet as a re-routed packet, and each node having means to modify the data field to increment the value on determination that no acknowledgement message has been received following transmission of the packet as a re-routed message packet. Thus, the indicator for the number of unsuccessful re-routed transmissions a message has had are carried within the message itself, and any device of the system can handle the message packet removal on detection that the data field has reached its maximum value.
The present invention also provides a device for use as a node within the above-described system, with the device comprising control means coupled with an address data store and a transceiver. The control means is operable to format data packets according to a predetermined protocol and transmit those packets via the transceiver, and to extract data from such packets received via the transceiver, wherein the device has an identifying address and is configured to ignore all received message packets not including the identifying address.